Polymethine dyes containing one or more chalcogenazolium nuclei have been widely employed in photography. These dyes have found particular utility as spectral sensitizers for silver halide photographic emulsions. Such emulsions depend on the ability of silver halide microcrystals or grains to form a latent image by absorbing light on imagewise exposure. Unfortunately, the native sensitivity of silver halide grains does not extend beyond the blue portion of the visible spectrum. To record longer wavelength green, red, or infrared (collectively referred to as minus blue) exposures, it is known to adsorb to the surface of the silver halide grains a spectral sensitizing dye capable of absorbing light of these longer wavelengths, thereby extending the latent image forming capability of the grains. In many instances the dye also reduces the sensitivity of--that is, tends to desensitize--the silver halide grains to a significant extent within the spectral region of native sensitivity.
In polymethine sensitizing dyes electromagnetic radiation absorption maxima are shifted bathochromically as the number of methine linking groups is increased. For example, monomethine cyanine dyes, those having a single methine group linking the basic nuclei, typically exhibit absorption maxima in the blue region of the spectrum. Cyanine dyes having absorption maxima in the green and red regions of the spectrum are typically carbocyanine dyes--i.e., dyes with three methine groups linking the basic nuclei; and cyanine dyes having absorption maxima in the infrared typically contain five or more methine groups linking the basic nuclei. Lengthening the methine chain joining the dye nuclei, though the most common approach to bathochromically extending absorption, suffers a notable disadvantage in that it also tends to increase desensitization.
It has been observed in the art that marked bathochromic shifts in absorption maxima occur that cannot be accounted for merely in terms of the chain length of methine linking groups. Such bathochromic shifts have been attributed to aggregation of the dye molecules. Aggregation producing a bathochromic shift, e.g. J aggregation, can be particularly advantageous on the silver halide grain surfaces. Among the most successfully employed aggregating dyes are carbocyanine dyes containing a chalcogen atom in one or more nuclei. It has been observed that substitution of the central or meso carbon atom of the methine chain linking the nuclei can markedly improve aggregation of these dyes. Chain substitutions of polymethine dyes also have utility in modifying other properties, such as oxidation characteristics.
It is known that a bathochromic shift in absorption maxima of up to 5 nm per nucleus can be realized when a selenium atom is substituted for a sulfur atom in a chalcogenazolium dye nucleus. Thus, with a symmetrical simple cyanine dye a bathochromic shift of the absorption peak of up to 10 nm can be realized by substituting selenium for sulfur in both nuclei.
Although Wilson U.S. Patent 2,323,503 extends generic ring formulae to include tellurazole nuclei as extrapolations of investigations of other chalcogenazole methylene color formers, the true state of the art is summed up by Middleton U.S. Pat. No. 2,339,094:
It may be observed that the difficulty of reaction resulting in the production of azoles containing members of the oxygen group of elements in the azole ring may vary greatly with different elements, becoming greater in proceeding from the non-metallic elements such as oxygen and sulfur to the more strongly metallic elements such as selenium and tellurium. This probably accounts for the fact that many of the oxazoles and thiazoles have been known for years while the preparation of most of the selenazoles has been accomplished more recently and some of them are still unknown although the corresponding oxazoles and thiazoles are known. Furthermore, the tellurazoles from the simplest to the more complex derivatives have not been described up to the present time.
While the art has heretofore been unsuccessful in preparing tellurazolium salts and their derivatives, it should be noted that divalent tellurium atoms have been placed in other ring structures. Benzisotellurazole-1,2 is described in "Un Nouvel Heterocycle Tellure: le Benzisotellurazole-1,2", by Campsteyn et al, Journal of Heterocyclic Chemistry, Vol. 15, August 1978, pp. 745-748. Unfortunately no derivative of benzisotellurazole-1,2 is disclosed. Without a 3 position substituent the ring structure is itself severely restricted as a possible photographic addendum. Further, in general isochalcogenazoles are less desirable and more infrequently suggested for use as photographic addendum than the corresponding chalcogenazoles, since the chalcogen to nitrogen bond in the ring is a potential source of instability.
Tellurium atoms have been incorporated in ring structures other than azole rings of various dyes. Japanese Kokai No. 136420, laid open Nov. 25, 1976, discloses a 1-tellura-3,5-cyclohexanedione nucleus in a merocyanine sensitizing dye in a silver halide emulsion. Detty et al U.S. Pat. No. 4,329,284 discloses 1,2-oxachalcogenol-1-ium salts, wherein the chalcogen can be tellurium or selenium, to be useful in photoconductive compositions. Detty et al U.S. Pat. Nos. 4,365,016 and '017 disclose tellurapyrylium dyes for use in photoconductive compositions.
Gunther et al U.S. Ser. No. 660,155, filed Oct. 12, 1984, titled PHOTOGRAPHICALLY USEFUL CHALCOGENAZOLES, CHALCOGENAZOLINES, AND CHALCOGENAZOLINIUM AND CHALCOGENAZOLIUM SALTS, commonly assigned, discloses for the first time procedures for obtaining tellurium atom containing heterocyclic ring structures useful for the preparation of polymethine dyes. However, Gunther et al contains no teaching directed to preparing such polymethine dyes with methine chain carbon atom substitution.